The cyano-containing phenoxybenzyl esters prepared by the process of the present invention are cyclopropane carboxylic acid esters and phenylacetic acid esters defined by the formula: ##STR1## in which R is an optionally substituted cyclopropyl group or an optionally substituted benzyl group. These compounds are significant in that they possess good pesticidal activity yet are low in toxicity with respect to warm blooded mammals. Such compounds are described, for example, in U.S. Pat. No. 4,061,664 incorporated herein by reference. Several methods are known for preparing these compounds including, for example, the method described in aforementioned U.S. Pat. No. 4,061,664 in which an aqueous solution of a carboxylic acid is neutralized and then contacted with a solution of a phenoxybenzyl halide in a water-immiscible base in the presence of a phase transfer catalyst. Other methods of preparation include (i) reacting an acid ester such as ethyl 3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate, also referred to as "DV-acid ester", with alpha-cyano-phenoxybenzyl alcohol in the presence of a transition metal alkoxide catalyst, (ii) reacting an acid halide such as "DV-acid halide" with m-phenoxybenzaldehyde in the presence of sodium cyanide and a phase-transfer catalyst such as tetrabutylammonium chloride, and (iii) reacting an alpha-cyano-phenoxybenzyl alcohol possessing an alpha-cyano structure with a cyclopropane carboxylic acid halide or a phenylacetyl halide to form the corresponding cyano-containing phenoxybenzyl ester. For example, 3-phenoxy-alpha-cyanobenzyl-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane carboxylate, also known as cypermethrin, (trademarked Ripcord.RTM. by Shell and Imperator.RTM. by ICI) can be produced by method (iii) by reacting 3-(2,2-dichlorovinyl)2,2-dimethyl-cyclopropane carboxylic acid chloride with alpha-cyano-phenoxybenzyl alcohol.
It has now been found that the cyano-containing esters of the present invention can be prepared by reacting an acid halide with an ester of an alpha-cyano-phenoxybenzyl alcohol. A particular advantage of the present process resides in the utilization of an ester reactant in lieu of an alpha-cyano-phenoxybenzyl alcohol reactant. In those processes requiring alpha-cyano-phenoxybenzyl alcohol, the alcohol reactant can decompose to the corresponding phenoxybenzaldehyde during storage and, to a lesser extent, during formation of the final pyrethroid product. This is undersirable since toxic hydrogen cyanide is liberated during transformation and product yields are decreased due to the loss of alcohol reactant during transformation. In the past, manufacturers and/or suppliers of the cyanohydrin reactant have sought to remedy this situation by adding acidic agents such as sulfuric acid or p-toluenesulfonic acid to the reagent prior to shipment to maintain the cyanohydrin in a protinated form. While this has proven to be reasonably effective, the addition of an acidic agent to the reagent increases the cost of the reagent to the user and must be removed from the final pyrethroid product prior to its application and use. Thus, a welcome contribution to the art would be the provision of an industrial process for the manufacture of alpha-cyano-phenoxybenzyl esters which does not require the use of an alcohol reactant and thereby avoids the problems of decreased pyrethroid production and hydrogen cyanide liberation.